scx_rustland_core: drop unused variable

[EricccTaiwan]

Remove the unused variable cpu from dispatch_task() in main.bpf.c. The variable was never read or assigned, and its presence triggered a -Wunused-variable warning during BPF compilation.

Log:

warning: [email protected]: main.bpf.c:548:6: warning: unused variable 'cpu' [-Wunused-variable]
warning: [email protected]:   548 |         s32 cpu;
warning: [email protected]:       |             ^~~
warning: [email protected]: 1 warning generated.
warning: [email protected]: main.bpf.c:548:6: warning: unused variable 'cpu' [-Wunused-variable]
warning: [email protected]:   548 |         s32 cpu;
warning: [email protected]:       |             ^~~
warning: [email protected]: 1 warning generated.

[EricccTaiwan]

scx_layered failed, which I believe shouldn't have occurred.

Log:
integration-test / integration-test (scx_layered, --disable-topology=false --disable-antistall)

Run actions/upload-artifact@v4
  with:
    name: scx_layered--disable-topology=false --disable-antistall_logs_150479[2](https://github.com/sched-ext/scx/actions/runs/15047926877/job/42295170105#step:21:2)6877_1
    path: ./log_save/*.ci.log
    compression-level: 9
    if-no-files-found: warn
    overwrite: false
    include-hidden-files: false
  env:
    SCHED_EXT_KERNEL_COMMIT: a6d8c4bc44959b[3](https://github.com/sched-ext/scx/actions/runs/15047926877/job/42295170105#step:21:3)d4d27ffd6364f9ec24f7418c6
    CACHE_ON_FAILURE: false
    CARGO_INCREMENTAL: 0
With the provided path, there will be 7 files uploaded
Artifact name is valid!
Root directory input is valid!
Beginning upload of artifact content to blob storage
Uploaded bytes 110012
Finished uploading artifact content to blob storage!
SHA256 digest of uploaded artifact zip is [4](https://github.com/sched-ext/scx/actions/runs/15047926877/job/42295170105#step:21:4)24613de578b70ca90a46243d01aae9ba47ab867aab[5](https://github.com/sched-ext/scx/actions/runs/15047926877/job/42295170105#step:21:5)60762a13617cdcf611e0
Finalizing artifact upload
Error: Failed to FinalizeArtifact: Unable to make request: ECONNRESET
If you are using self-hosted runners, please make sure your runner has access to all GitHub endpoints: https://docs.github.com/en/actions/hosting-your-own-runners/managing-self-hosted-runners/about-self-hosted-runners#communication-between-self-hosted-runners-and-github